Just thought I'd close out this query.
Some information I received last week dies not explain why they married twice, but it at least confirms that it was the same couple.
In a diary kept by Clarissa's uncle, he recorded that Clarissa Elziabeth Hardess married Robert Parkinson on 6 Feb 1815 in St. Katherine Coleman.
Furthermore, their first child was born at 172 High Holborn in St. Giles in the Fields.
Thanks to you all,
Justin